The resulting silicon anode retains the 3D hierarchical architecture of the reed leaf, and features an interconnected porosity and inside carbon coating. The anodic material exhibits high specific capacity, very good rate capability, and cycling stability, just as it is required in advanced lithium-ion batteries. Even after 4,000 cycles and at a rate of 10C, the anode achieved a specific capacity of 420 mAhg^-1.

A paper describing their work is published in Angewandte Chemie International Edition.

a) Flow chart showing conventional synthesis routes of nano-Si for Li-ion battery anodes, along with of the sustainable and low-cost synthesis route from natural reed leaves. b) Detailed flow chart of the process for 3D porous Si-C anode from natural reed leaves to the 3D porous SiO2 precursor and finally the 3D porous Si-C anode. Source: Liu et al.Click to enlarge.

While a manifold of silicon-based nanostructured anodes with very good electrochemical performances have been successfully fabricated, most of them lack potential of practical application owing to the high cost of precursors and methodology or the inability to produce materials at gram or even kilogram level.

… Plants can absorb silicon in the form of silicic acid (Si(OH)₄ or Si(OH)₃O^-) from the soil. The ability of a plant to accumulate silicon varies greatly from species to species (0.1–10 % of shoot dry weight). Silicon accumulation exceeding 4% is especially common in monocotyledonous plants, and hence in the plant families of Poaceae, Equisetaceae, and Cyperaceae. Reeds, as the typical members of the Poaceae family, grow in along rivers, or in shallow water near ponds. They are widely distributed worldwide in the wetland of the temperate regions. As a living plant, reeds absorb silica from soil, and the silica accumulates around cellulose microcompartments. Therefore, reeds are suitable natural reservoirs of nano-structured silica and its derivatives. Yet they are not only appropriate Si sources, they also contain silica in a very favorable nanoscale arrangement.

… reed leaves exhibit well-defined sheet-like 3D hierarchical micro- structures, which as we demonstrate can be transformed into a well-suited 3D highly porous hierarchical Si architectures.

—Liu et al.

Synthesis is based on magnesiothermic reduction of 3D porous SiO₂ converted from natural reed leaves. It utilizes the reed leaf as skeleton template, and the in situ generated MgO by-products as pore template.

Compared with the reported methods of producing nanostructured Si anodes, the reed-based method offers a number of advantages, the researchers said:

1. reed leaves are sustainable materials source;
2. the recovered silicon retains the favorable 3D silica nanostructure of the reed leaves (which allows for superior battery performance by mitigating pulverization);
3. the overall method is simple; and
4. the overall process does not use expensive Si precursors or reagents.

Electrochemical performances of highly porous 3D Si-C nanonet anodes. a) CV curves at a scanning rate of 0.1 mVs-1 in the voltage range of 0.01–1.0 V. b) voltage–capacity curves at 0.05 C and 0.5 C rates. c) Rate capability at different rates (increased from 0.2 C to 20 C after the first three CV cycles). d) Cycling performances at 0.5 C rate (first activated with two cycles at 0.05 C). e) Long-term cycling performance of the highly porous 3D Si-C nanonet electrode at a current density of 10C. Liu et al. Click to enlarge.

The topological architecture of the original silicates within the reed leaves is extraordinarily well preserved during the applied chemical and physical treatment steps. Upon the purification from the dry reed leaves, the three-dimensional structure only shrinks, but retains its mesoporous network. It does not even change during the reduction to the final carbonized silicon network.

The magnesiothermic reduction has two advantages. First it results in a silicon microstructure retaining the original silicic structure. Second, etching of the MgO inclusions leads to a high internal pore density. These features together with the carbon coating of the silicon leads to the attractive electrochemical performance for Li-ion batteries, such as large reversible capacity, high rate capability, and superior cyclability. The 3D hierarchical architecture and 2D highly porous nanosheet/nanonet units buffer the huge volume change, reduce the diffusion-induced stress, and facilitate the diffusion of Li ions and electrolyte into the electrode. The surface carbon coating enhances not only the overall electronic conductivity of Si but also mechanically stabilizes the whole 3D porous structure. Moreover, given the sustainable and facile nature of the synthesis procedure, the described 3D porous Si-C nanocomposite has a great potential as a practical anode material for Li-ion batteries.

—Liu et al.

Resources

• Jun Liu, Peter Kopold, Peter A. van Aken, Joachim Maier, and Yan Yu (2015) “Energy Storage Materials from Nature through Nanotechnology: A Sustainable Route from Reed Plants to a Silicon Anode for Lithium- Ion Batteries” Angew. Chem. Int. Ed. 54, 1 – 6 doi: http://dx.doi.org/10.1002/anie.201503150